It’s been quiet here lately, but I thought this would be of interest:
It’s been quiet here at the Catalytic Engineering blog, as most of our time is being spent on a long term project for Facebook, performing system modelling and energy analysis for its Connectivity Lab program to deliver Internet to poorer and isolated parts of the world, using HALE (high-altitude, long-endurance) unmanned aircraft.
Keeping aircraft in the stratosphere 24 hours a day, 365 days a year is a fascinating problem of maximizing solar energy during the day, and storing the energy for flying overnight with high round trip efficiency and high energy density (Wh/kg). It’s well aligned with the energy storage and system optimization work that we’ve done in the past.
It’s a great, highly-motivated team and the goal of trying to bring connectivity to the next billion people is exciting. Unfortunately we can’t say anything about the details, but here’s some public information.
And – we are hiring for selected positions in the UK and California, mainly intern positions for now.
I’m at Fuel Cell Seminar 2015 in sunny Los Angeles and chairing a session on flow batteries for energy storage (#fcs2015). There’s a Toyota Mirai fuel cell car and a few hundred fuel cell people here.
My favorite quote so far is from Jon Slangerup of the Port of Long Beach (paraphrased):
California does it first.
Always has, always will.
We go first, we take the lumps, and other societies follow.
Another quote was a speaker praising a 91 year old fuel cell industry leader, saying roughly: he showed great strength and determination, whether in flying over and bombing Stuttgart, or leading fuel cell development. (This was while I was discussing membrane development with a entrepreneur… from Stuttgart. And half the German companies working in fuel cells are based there.)
Here’s the schedule. Let me know if you have any questions about the conference.
Like syrup and water
Next time you’re at a fast food restaurant, look closely at the soda fountain. You may be able to see that the stream of Coca-Cola that comes out is actually a stream of syrup mixed into a stream of carbonated water, dispensed “on the fly” at the nozzle. Controlling that ratio is important; too much syrup and you get an over-sweetened mess. Too little syrup and you may as well be drinking mineral water.
In much the same way as the soda mixer, it is possible to mix hydrogen into the natural gas pipeline network. This is a way to absorb excess solar and wind energy: surplus renewable electricity is put into an electrolyzer to create hydrogen, which is temporarily stored in buffer tanks, and then injected into a natural gas pipeline’s flow of gas.
The ratio is important – for maximum energy storage, as much hydrogen as possible should be put into the natural gas. But as the ratio increases, problems appear: hydrogen can leak out of normal pipeline metals, the ignition properties of home boilers and utility gas turbines are affected, and the value of each cubic meter of gas goes down (less energy, and therefore dollars, per volume).
So how much energy could you store in a natural gas system? There have been some articles that quote Germany’s natural gas reserve energy capacity as 200 TWh, which would dwarf all the hydroelectric energy storage in the world put together (a TWh is a terawatt-hour, enough energy to run a city of 200,000 for a year.).
But this natural gas energy is very different from the amount of electricity that could be stored. Let’s have a closer look.
If batteries are 95% efficient, and therefore hardly generate any heat, why do they sometimes suffer from overheating and shortened lifetime? And why do fuel cell cars like the Toyota Mirai (pictured above) have such huge radiator grills? To explain these questions we’ll take a very simple look at efficiency and heat transfer. To a scientist “efficient” is basically synonymous with “doesn’t produce much waste heat”, and all waste heat needs to be sent into the environment (rejected) to avoid overheating.
Let’s use cars as an example.
The following graph compares three hypothetical vehicles, each of them delivering the same 100 kW (133 hp) of brake power. The combustion engine requires the input of 500 kW of gasoline chemical energy, converts 20% of it to mechanical power, and loses the other 80% as waste heat. The battery vehicle uses about 118 kW of battery chemical energy to deliver the same 100 kW brake power, so it only needs to get rid of 18 kW of heat. A fuel cell system is about 50% efficient so rejects 100 kW of heat.
Why does the battery vehicle have any heat issues if its produces so few kilowatts of waste heat? The reason has to do with the fact lithium-ion batteries have a relatively narrow operating range. While combustion engines can handle temperatures from -30°C to 110°C, and fuel cells about -20°C to 80°C, Li-ion batteries should not be run hotter than 55°C. Higher temperatures shorten battery life and reduce performance.
The difference between 55°C, 80°C, and 110°C doesn’t sound like a lot. But the problem comes on hot summer days, where the ambient air cooling the engine, fuel cell, or battery may be 40°C or hotter. As engineers know, it’s temperature difference that transfers heat. The difference between a hot engine at 110°C and 40°C is substantial, but a 55°C battery is only 15°C hotter than the environment, which means battery electric vehicles still need effective cooling systems (high surface area radiators, good fans, etc.). To show this visually, the colored area on the graph below is the available temperature difference for each of these technologies on a hot 40°C day:
Fuel cells have the same issue – only 100 kW of waste heat, but it needs to be rejected across a 40°C temperature difference, which is why fuel cell cars have such large radiators and air intakes despite having only a quarter of the waste heat of a combustion engine. (And that’s why a lot of work goes into developing special radiators and higher-temperature fuel cells).
This brief dip into thermodynamics explains why efficient technologies like batteries and fuel cells may still require a great deal of cooling. But it’s just the tip of the iceberg (so to speak) for cooling of advanced power systems, though – there are many other interesting topics:
- When to use air cooling vs. liquid cooling
- The effect of solar heating on a parked car or battery backup system. 1 kW/m^2 of solar energy can be a significant heating load
- The amount of waste heat that is carried away by a combustion engine’s exhaust gases
- How to heat a fuel cell or battery from -40°C to its minimum operating temperature
- How to trade off insulating a fuel cell or battery from cold weather, vs. the risk of overheating during hot weather
Bonus chart – same data as above but in a single graph. The area of each circle is proportional to waste heat.
I’m constantly re-creating a version of this tool every few years when I design fuel cells and flow batteries, so I decided to make a free, user-friendly public version. It’s a simple Excel spreadsheet that can quickly and easily iterate different fuel cell designs, and to understand test results. For example, it can:
- Calculate what reactant flow rate is needed to feed a stack adequately, and how much blower parasitic power that flow will cost
- Calculate what coolant flow rate is needed to keep the stack temperature stable
- Estimate humidification and predict exhaust gas water vapor content (this is my favorite as it strongly affects water balance and condenser design, and it’s not a straightforward calculation.)
It’s not a full-blown microscopic model or CFD simulation, but it’s simpler to use and it deals with heat, mass balance, and electrical power. Can be extended to electrolyzers, flow batteries, and metal-air batteries.
Mainly Japan, but some recent articles suggest that it’s also South African mining companies:
LONDON, May 29 (Reuters) – Platinum miners betting on fuel cell vehicles to help boost demand for the precious metal and lift moribund prices are in danger of having their hopes dashed, at least in the medium term: electric and hybrid cars are taking a bigger share of the market …
Anglo American Platinum Ltd. is offering to help investors set up South African production of hydrogen fuel cell-based products, which use the precious metal as a catalyst to generate electricity, as a way to encourage demand …
Hawaii – a small, isolated group of islands in the middle of the Pacific – is served by a small, isolated group of electricity grids in the middle of the Pacific.
With solar on 10% of ratepayers’ rooftops (and rising!) it’s been described as a testbed or “postcard from the future” where grids run in the framework of high-renewables, high-intermittency infrastructure. One reason is that Hawaiian electricity prices are expensive, as almost three quarters comes from burning petroleum that has to be shipped in. Current prices are 35-45 cents/kWh, depending on the island. This is 3X the mainland price and even higher than the typical German pricing in our last article. However, the case for energy storage is mainly about regulations and grid stability, rather than cost and energy storage engineering.
Local utility HECO had earlier allowed net metering (“NEM”), effectively letting residents sell solar at close to retail prices. Where net metering is in effect, there’s no economic incentive to store your own solar electrons for self-consumption. However, the rapid adoption of solar and the implied subsidy to PV owners has caused HECO to try to roll back NEM, and propose much lower payments (closer to 15 cents/kWh). (Needless to say, there’s a Godzilla-vs.-Mothra style battle going about this proposed change.). This shifts Hawaii closer to the German model discussed last post, where self-storage starts to make sense.
In the last few months the biggest issue seems to be that HECO has slowed approvals for solar installations, claiming that too much distributed generation was overloading its circuits. Here the storage case is stark – if you’ve already paid tens of thousands of dollars for a solar roof based on net metering being in place, but the utility isn’t allowing you to earn revenue by feeding back to the grid at all, then energy storage could be very attractive at even a high price.
The so-called “non-export” option might get around this. Residences would produce solar energy for self-consumption, store energy when there was a net surplus, and draw power from the grid when the battery was depleted. Your system might deliberately dump power when the battery was full but surplus solar was still being generated. This way you would never put power back onto the grid (addressing some of HECO’s concerns) while maximizing your own use of your own solar power. HECO is running studies for this option, and also for so-called “smart export” – see page 29 here
Unfortunately utilities are very conservative – what monopolies aren’t? — and tend to demand a lot of testing before they’re willing to certify new products (in this case, equipment to ensure your electronics will never put power back onto the grid). So it may be easier to just go fully off-grid while you wait. That’s why SolarCity is telling Hawaiians “Don’t let anyone stop you from going solar. Installations expected to begin in the first half of 2016.”
In energy storage terms, you might need a few 7 kWh Powerwalls to go fully off-grid and store up energy for evenings avoiding all HECO worries about backfeeding. Hawaii’s residential customers average about 6,000 kWh per year (16 kWh per day), so a crude estimate would be three Powerwalls (storing all energy at times of high solar energy but zero consumption, and consuming it all at times of zero solar energy). Cost could be as low as $13,160 (three Powerwalls and one new inverter). Lead-acid battery solutions are already common.
In this case, roughly speaking the incremental cost of adding battery backup to a solar PV system could be $0.22/kWh, assuming the Powerwall can sustain daily cycling of 7 kWh from beginning to end of its 10 year lifetime. This needs to be added to the cost of the solar PV system. Michael Roberts of the University of Hawaii has a more detailed analysis of the costs of PV that includes subsidy.
Going off the grid as SolarCity proposes is bold but not ideal – the grid is amazingly useful as a huge reservoir of electrons that you’d probably only disconnect as as last resort. Your daily energy usage would have to be low – and the daily solar insolation very very reliable – to make Powerwalls worthwhile. But until the HECO logjam is freed up (either by changing policy or improving the infrastructure) it may be a tempting option for some Hawaiians. And more and more ‘grid defections’ could mean big trouble for HECO.
The Hawaiian case foreshadows the grid stability issues coming (one decade or another) to the rest of us. It also illustrates how utilities are being caught between the irresistible force of rapidly-ramping residential solar, and the immovable object of still-being-amortized, pre-PV infrastructure investments. And as with all sudden ecosystem shifts, the survivors will be the ones who can adapt to the new realities – while the others fade to black. (Though in financially speaking, “fade to red” might be more appropriate.)
One final parting thought is that the challenges facing Hawaii’s grid today, and ours tomorrow, can be solved on both sides of the meter. While Tesla’s Powerwall gets all the attention, the utility-scale Powerpack is the real breakthrough, with its $250/kWh pricing. We’ll un-”pack” that in a later article.
image credit (C) Ralph Larmann for the LICHTGRENZE project, tracing the path of the Berlin Wall in light – used with permission
By Bruce Lin with Matthew Klippenstein
We continue our analysis of Tesla’s stationary energy storage business with a closer look at the German residential market, and provide a tool for you to try your own numbers. (Part 1 is here).